With application of various communication network technologies, each of recently released terminals includes a communication module for supporting a plurality of communication networks. According to the related art, a terminal using a Global System for Mobile communication (GSM), a Universal Mobile Telecommunication System (UMTS), or a Long Term Evolution (LTE) networks among the communication networks has inserted therein a Subscriber Identity Module (SIM) card including a user's personal information. The terminal obtains user information from the inserted SIM card. When the user is determined as a rightful (e.g., authorized) user registered in a network, the terminal performs normal operations.
Therefore, when the SIM card has not been inserted in the terminal using the communication network, the terminal cannot obtain subscriber information and, consequently, the terminal cannot perform a normal communication function. Therefore, the terminal according to the related art is designed so that the terminal can perform an emergency call in preparation for an emergency case. For example, even when a SIM card is not inserted in the terminal according to the related art, the terminal may perform an emergency call. Similarly, when the terminal according to the related art is in a Personal Identity Number (PIN) lock state and a PIN Unlock Key (PUK) lock state, the functions of the terminal are limited. According to the related art, the terminal searches a cell of a network which the terminal can support in order to find an optimal channel for transmitting an emergency call signal. Such a process of searching the cell of the network is referred to as an RAT search.
The RAT search according to the related art is performed in all communication networks for which a terminal can support. This process is illustrated in FIG. 1.
FIG. 1 is a flowchart illustrating an example of the RAT searching method according to the related art.
Referring to FIG. 1, a case in which a terminal supports LTE, UMTS, or GSM communication networks is illustrated.
The terminal performs the RAT search according to a predetermined search sequence. When a SIM card has not been inserted into the terminal, the terminal performs the RAT search according to an RAT priority stored in the terminal. When the terminal is in the PIN lock state or the PUK lock state, the terminal performs the RAT search based on information of Registered Public Land Mobile Network (RPLMN) and Registered RAT (RRAT) which are stored in the SIM card. When the RAT priorities stored in the terminal or the SIM card are in the sequence of the LTE network, the UMTS network, and the GSM network.
In operation 101a, the terminal searches for an LTE network. If the terminal discovers an LTE network at the first search in operation 101a, then the terminal proceeds to operation 101d in which the terminal camps on a cell of the LTE network.
In contrast, when an LTE network is not discovered in operation 101a, then the terminal proceeds to operation 101b in which the terminal performs the RAT search a UMTS network.
If the terminal discovers a UMTS network in operation 101b, then the terminal proceeds to operation 101d in which the terminal camps on a cell of the UMTS network.
In contrast, when the terminal does not discover a UMTS network in operation 101b, the terminal proceeds to operation 101c in which the terminal searches for a GSM network.
If the terminal discovers the GSM network in operation 101c, then the terminal proceeds to operation 101d in which the terminal camps on a cell of the GSM network.
In contrast, when the terminal does not discover the GSM network in operation 101c, the terminal returns to operation 101a. 
Meanwhile, the LTE network employs a Packet Switching (PS) scheme. However, in the PS scheme, a voice telecommunication is not supported so that an emergency call is impossible. Accordingly, when the terminal camps on an LTE network, the terminal should re-perform the RAT search for a communication network other than the LTE network, for the emergency call. The time required for the re-performing of the RAT search hampers the usefulness of an emergency call prepared for an emergency situation.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.